Manufacture of pulp



C. K. TEXTOR MANUFACTURE OF PULP Dec. 3, 1935.

Filed sept. 1e, 192e LMSSQ bss `Patented Dec. 3, 1935 UNITED STATESPATENT OFFICE MANUFACTURE F PULP l l Clinton K. Textor, Cloquet, Minn.,ass'ignor to Northwest Paper Company, Cloquet, Minn., al

corporation of Minnesota Application september 1s, 1926, sensi No.135,930

5 claims. (01.92-1'1) This invention relates'to improvements in wooddigestion with normal suliites,` the pulp produced, the recovery of thechemicals in the resulting liquor, and the preparation of the cookingliquor. woods in general, the present application is directed moreparticularly to the preparation of pulp from woods, such as poplars,gumwoods, maples, oaks, elms, basswood, and all such woods as aregenerally included under the term hardwoods, and loosely characterizedas short-- with sodium sulte liquor, the regulation of the alkalicarbonate in the cooking liquor to minimize the corrosionof theequipment without adversely affecting the economy or the efficiency ofthe process, and the treatment of the so-called black liquor, resultingfrom the cooking operation so that it may be highly concentrated ineicient tubular evaporators without plugging these evaporators byseparation of solids.

Other objects and advantages of my invention will become apparent tothose skilled in the art from the following specification. Theaccompanying gure shows, more or less diagrammatically, one arrangementof apparatus capable of carrying out the process of this invention, butit is to be understood that this illustration is given only for thepurpose of'facilitating an explanation ofthe process, and it is not tobe construed as a limitation of the invention short of its scope ascomprehended in the appended claims.

In 1880, Cross made known the utility of sodium sulfite as aWooddigestant. Since that time a number of modifications have appearedin the literature and in patents of the United States, Germany, Canadaand Great Britain. This art shows that it is consideredrnecessarywhenusing an essentially sodium sulte cooking liquor, comprising,principally, the normal sodium salt (NazSOa), to have present causticsoda (NaOH) to dissolve 'the silicates and aluminates found in cellulosebearing materials, such as wood. Schachts German Patents Nos. 122,171and 131,118 specifically require the presence of small Although theseimprovements apply to amounts Iof caustic soda. Other investigators havedeemed it necessary to have still larger amounts of caustic soda presentin the normal sodium sulte cooking liquor, especially when highlyresinoufsl Woods are used. I havefound 5 that it is not necessary tohave caustic soda (NaOH) present in such a sulte liquor to produce asatisfactory pulp, and that there are certain advantages in operationand certain improvements in the pulp resulting directly from replacingthestronger caustic soda of the prior art with a milder alkalicarbonate,l such as sodium bicarbonate, or sodium carbonate.

By my method of cooking, which is described in detail hereinafter, Iobtain a high yield or well cooked pulp, in some cases as' high 60% (forshort-bered wood) Vof the weight of the wood employed, both beingcalculated on oven driedV material. The product thus produced is ofspecially good strength, has an unusuallyy White color for unbleachedpulp, and possesses those characteristics essential for the manufactureof strong, durable paper of the highest grade. The pulp is readilybleached to a good white color,

the bleached pulp retaining practically all of the ture and theproportions of material to wood may be varied according to the type ofwood, the moisture content of the wood, the kind of pulp desired, andother factors hereinafterdescribed as carried out with aspen.

When cooking a short-bered wood, such as aspen, I prefer to use thefollowing procedure. The digester I is filled to the desired height with40 aspen, chipped to the usual size, and the cooking liquor is added involumes similar to those employed for cooking wood by the soda or modied-soda process, as is well known to those versed in the art. I find itdesirable to add from one-quarter to three-quarters cubic footof cookingliquor per cubic foot of chips as they lie in the digester, dependingupon the moisture content of the wood, the method of circulation, themethod of heating, and other factors. The preferred cooking liquor is ofsuch a concentration that there is added for every pounds of oven-driedchips from 30 to 40 pounds of sodium sulte (NazSOa) and 1.3 to 2.6pounds of sodium bicarbonate (NaHCOs). When it is necessari? to dilutethe l cooking liquor for use in the dlgester, a waste black liquor, moreparticularly described hereinafter, or black liquor washings may be usedin place of Water Without any detrimental result. The detailed method ofpreparing the cooking liquor either as a fresh solution or as aregenerated solution is stated in detail hereinafter in the ensuingdescription. Such a cooking liquor has been found to have an alkalinitycorrespondingv bonate to produce the desired safeguard againstexcessive` corrosion. In using greater quantities of carbonate, noadditional advantages have been observed. It vis important that thecarbonate used be in addition to the amount of sodium sulte required andnot in substitution therefore. The quantity of sodium sulte considerednecessary for the process is deemed otherwise important in at least oneparticular circumstance.' I have found that the preferred liquor in thequantity stated must contain sucient sodium suliite (NazSOs) to preventthe production of black or dark brown pulp by any sodium thiosulfate(NazSzOs) which may be present as an impurity, as hereinafter explained.It appears that if the normal sulte becomes exhausted during thedigesting process, the thiosulfate then acts predominantly upon the pulpdisadvantageously, producing a dark colored product.

The cooking process is preferablycarried out in an unlined steeldigester by a procedure similar to that of the soda or modif-led sodaprocess. By a soda process, I refer to one which employs caustic alkali(NaOH) as the pulping agent, while by a modified soda process I refer toa soda process in which the liquor consists chiefly of caustic alkalibut to which either sodium sul. fide (Na2S) or sodium sulte (NazSOs) orother materials have been added. A temperature of 175 to 188 C. may beemployed and the corresponding saturated steam pressure is maintainedwithin the digester. With the above quantities of materials, thedigester is heated directly, steam being injected thereinto. By asuitable change in proportions, the digester may be heated indirectly,if desired. The time of cooking varies from three to seven hours, beinggenerally shorter for this type of wood than for coniferous woods. Thefirst one or two hour period is used for attaining the desiredtemperature and pressure. This period is also considerably shorter thanwhen the same process is used for coniferous woods,\and this is believedto be a result of the more rapid sorption of liquor by various woodswhich are classed with aspen. The digester is relieved ofnon-condensing, gases throughout the cooking operationeitherperiodically or continuously. At the end of the cooking operation,the digester is blown through a blowline 2 into a receiving vessel 3. f

The next step in the process is to free the pulp from the liquor,resulting from the digestion, this liquor hereinafter being referred toas black liquor. The separation is herein accomplished by means of acontinuous filter, indicated at 4,

from which the pulp is discharged in one direction and the black liquorin another. The pulp is then subjected to a thorough washing andscreening operation, as indicated generally at 5, to remove undigestedpieces of wood and to free 5 it from traces of the discoloring blackliquor. Thorough Washing lis essential for the production of a pulp witha good natural color which is easybleaching. This wmed pulp may, ifdesired, be used for paper without further treatment, as in- 1o dicatedat 6. A bleaching operation is represented at 1, and the subsequentwashing at 8, after which the pulp is suitable for a high grade paper,as shown at 9. The unbleached pulp is of a color almost white and asgood as some coml5 mercially bleached pulp now found on the market. Itis much lighter and stronger than when a pulp is made from the same woodby the soda or a modified soda process. It has a color lighter thanunbleached spruce sulte pulp and when 2O made into a sheet it has abursting strength equal to spruce sulte pulp. By a sulfite pulp, I referto one made by an acid sulte solution having from 4% to 6% total sulfurdioxide in the solution of which 1% to 11/% is combined with the base,25 such as sodium, calcium or magnesium, the remainder being present asfree sulfurous acid.

The pulp of my process has been only slightly acted upon by the cooking,and is readily bleached toa brilliant white with slightly less bleaching30 powder than is required when the same wood is pulped by the soda ormodified soda process.

Another feature of the process is the cyclic operation as regards thebase metal of the digestion liquor. Sincesodium isy by far the cheap- 35est and the preferred base, the process'will be explained in detail withreference to the said liquor. In general, the preferred steps ofregeneration comprise concentration of the black liquor resulting fromthe digestion, incineration to burn 40 out the organic matter, leachingof the ash from the incineration, giving substantially a sodiumcarbonate (NazCOa) solution, and sulfiting of the leach liquor withsulfur dioxide (SO2) to form sodium sulflte (NazSOs) and to leave apredetermined amount of unsulted soda, preferably as sodium bicarbonate(NaHCOa).

In the concentration of the black liquor, I have used a quadruple effecthigh speed tubular evaporator in which the upper part of the body of theliquid being evaporated is contained in the lower ends of a `series oftubes, as represented in the drawing at I0. Considerable trouble hasbeen experienced in so evaporating this liquor because it depositssolids during the concentration. The solids deposited are, primarily,organic compounds which adhere to the tube walls and gather on'thebottom'of the evaporator, making it necessary to clean the latterseveral times a day.

Such a condition prevents continuity in concentration and is generallyvery undesirable, whatever type of evaporator is used.

I have found that the addition of a caustic al kali, such as sodiumhydroxide (NaOH), to the black liquor prevents the deposition of suchsolids and permits continuous evaporation to be carried out in thesystem described to a concentration of 36 to 40 B. (45% to 55% solids)"measured at 60 F. The evaporator can thus be operated continuously forseveral weeks as compared to a few hours. The addition of alkali may bemade just prior to the entry of the black liquor into the evaporator,rat the point I I in the drawing. l

The addition of caustic alkali leads to numerlindicated at u. I havetoune that sawdust ls ous unexpected advantages. It is customary in mostcyclic processes to supply at some point fresh' a general practice toemploy a waste liquor from, vsome other process whichv will provide therequired caustic soda, such, for example, as the black liquor from somesoda process of pulp formation.

The eect of the caustic soda isA to increase the alkalinity of the blackliquor to be concentrated. Tests on several black liquors show anincrease in alkalinity from pH 8.5 to about pH .9.2 upon addition ofcaustic soda through the medium of'bla'ck liquor from 'a caustic processand a further increase to pH 9.6 when additional caustic alkali (NaOH)is added to the mixed black liquors. The amount of caustic to be addedis immaterial so long as a sufficient amount is added to avoid thetroublesome conditions above described. The result desired is the guideto the amount required and this varies with the type of` wood for whichthe liquor has been used, the

time of cooking', and other specic conditions.

The syrupy mass or concentrate from the evaporation may be dried or rundirectly to an 'incineraton `various types of which are well known tothe art. Since the organic content of the concentrate is sufficient toprovide, upon combustion, the necessary heat to operate the mill, it isquite important that it be in the socalled soluble condition eilectedbythe caustic condition. The alkaline concentrate, acting as a. vehiclefor the organic matter, permits it to be pumped and conducted by pipelines to the incinerator, whereinto it may be sprayed as a liquid fuel.This not' only increases the eiiioiency of the equipment, but it alsoeliminates the necessity of handling the solids which otherwiseWouldsettle out on concentration, the danger of liquor concentrate is topass it through an incinerator I2 without approaching the fusion state.This results in an ash which is primarily sodium carbonate with somefree carbon, depending in amount upon the extent towhich it has beenburned. Furthermore, it may contain sodium suliide '(NazS) formed by the.reduction of certain of the sulfur compounds and small amounts of othersubstances. 'I'he incinerator ash is leached, as indicated at I3, withwater or with some suitable solution. which. is to be enriched in sodiumcarbonate (NazCOs) 'I/'he leach liquor is then ltered through a suitablelter, as

is a most satisfactory filtering medium. It has the advantageousproperty of' removing suspended solids, chiefly carbon, withoutclogging. It is more eiiicient than sand in this respect, being 6,lighter, more porous, and has a higher specific surface. I have used thesawdust ina bed to a depth of about three feet, the sawdust bed beingconfined or weighted down, as at I6, to guard against floating whenimmersed. 'I'he tendency 10 of sawdust to float, being greater than thatof sand, causes the bed to rise or expand rather than to pack when usedin a tank with a drainer bottom Il.

In practice, I have found it economical and 15` otherwise 'beneflcial toprepare the filtered leach liquor from the ash of the concentratedcombined black liquors obtained both from the di` gestion step of thepresent invention and from the old and well known caustic soda digestion20V procesces. In Table I, following, there are shown analyses of twosamples, designated A and B, each being a composite of forty samples ofsuch leach liquor.

Table I Pounds per cubic loot Sodium carbonate 13. 2O l 12. 75 Sodiumsuliite 37 44 Sodium thiosulfat 21 l5 S um sulfide... 1. 09 1. 43 Sodiumsulfate l. 20 1. 04 a5 Working under ideal conditions of incineration,all sulfur would be roasted out leaving a pure sodium carbonate ashwhich would yield an ideal leach liquor containing only sodium carbonate(Nazcoa): Practically, such an ideal 40 condition is not attainable,and, furthermore, it is not neces ary. However, when desired the eiectof these impurities m-ay be minimized by a special treatment of theleach liquor when it is, for any reason, desired toapproach the idealcondition. The sullting of the leach liquor with sulfur dioxide (SO2)initiates a series of reactions forming sodium thiosulfate (NazSzOs)from sodium sulfide (NazS). This formationof sodium thio-ulfate may beprevented by treating the leach liquor with carbon dioxide (CO2) taken,for example, from flue gases forming hydrogen sulfide (HzS) and sodiumbicarbonate. The nor-- mally gaseous but water-soluble hydrogen sulfideis retained by the soutien being treated. This condition effects acounter-reaction requiring the use of a considerable excess of carbondioxide to remove the hydrogen sulfide (HzS) from solution and therebyto permit continued conversion of the sodium sulfide into thebicarbonate. As

- there are large amounts of carbon dioxide available from flue gases,the excess required is no practical disadvantage to this operation froma chemical standpoint. From the standpoint of mechanical operation,however, it is possible to view this carbon dioxide treatment as anextra operating step in the regeneration. Such is not the case, for Ihave found it posfible to combine this step with the sulting step.

The leach liquor containing the sulfide is run against a counter-currentof gas containing sulfur dioxide. and carbon dioxide, the latter beingvpreferably in excess, so that substantially all the sulfur dioxide'isabsorbed whereby to permit carbon dioxide to flow on.' Thus, the excessof 5 asf carbon dioxide meets the incoming'leach liquor, whichat thisstage maybe rapidly re-circulated in the path of the carbon dioxide. Thecarbonic acid gas converts both the sodium sulde and the sodiumcarbonate to bicarbonate, and removes the resulting hydrogen sulfide.The sodium bicarbonate then absorbs the sulfur dioxide forming thenormal sulte, the extent of this reaction being controlled to form .thedesired composition of the emergent solution.

In the drawing, this treatment is represented as optional by dottedlines. `.4in-absorption tower I8 receives the leach liquor at the topand a stream of mixed gases enters the bottom of the tower. These gasesare preferably taken from the incinerator I2, thus to eiect asubstantial recovery of the .sulfur employed in the process, and anadvantageous use of the carbon dioxide from the combustion of the blackliquor concentrate.

The better practice, however, is to so effect the incineration of theblack liquor that the impurities occurring in the leach liquor may beneglected. In such a case, the leach liquor is sulted directly bypassing sulfur dioxide gas into the absorption tower I8, as representedat I9, until the liquor exhibits a predetermined composition of sodiumsulflte (NazSOa) and of unsulfited soda. A part of the original sodiumcarbonate will not be sulfited, but it will be acidied to sodiumbicarbonate by action of the sulfurous acid (H2503), formed upon thesolution of the sulfur dioxide gas in the water. When the bicarbonatecontent is in the region of 0.50 to 0.75 pounds per cubicfoot ofsolution, the sulting is deemed complete for the preparation of thepreferred cooking liquor, above described, and it is, consequently, leadto a storage tank 20.

For the purpose of explaining more 1n detail the various changes whichoccur during sulting, the following reactions are given:

In view of the above and of the preceding description, it should beunderstood that all the reactions are allowed to go to completion,except reaction 4. I'he supply of SO2 forming HzSOs by reaction 1 is outoff sufliciently early to leave the required amount of sodiumbicarbonate (NaI-1G03) formed by reaction 3, and to prevent its beingcompletely sulted by reaction 4.

The above described operation of sulting results in a suitable suli'ltecooking liquor for use in the digesting step of this invention. Table 1Igives actual analyses of two samples, C and D,

" of such cooking liquor, each being a composite of forty samples.

Such a cooking liquor containing impurities, as

above displayed, is quite suitable for the cooking of wood by my method.It has not been observed that impurities to the amount above stated haveany appreciable eiect upon -the quality of the pulp produced over thatwhich 4is produced in. the absence of such impurities. No claim is madeto the presence of these contaminating materials,

and it is to be understood that they are merely 5 incidental to apractical operation of the process hereinafter more particularly definedin the appended claims.

It should be explained at this point that heating a bicarbonate solutionwill cause reaction 3 10 to proceed in the reverse direction formingcarbonio acid which breaks down into Water and gaseous water-solublecarbon dioxide (CO2) Since sodium bicarbonate is used in the digestionand is under heat, it is evident that it breaks down 15 forming carbondioxide'and sodium carbonate (NazCOa). The carbon dioxide is releasedfrom the digester from time to time along with other gases formed in theprocesses. It is diflicult then to ascertain the ratio of normalcarbonate to bi- 20 carbonate during the cooking process. The escape ofthe acid constituent (CO2) during digestion thus permits an increase inthe alkalinity due to sodium carbonate (NazCOs). Such an increase hasbeen observed, as will hereinafter be 25 explained.

The cooking liquor containing essentially sodium sulfite and sodiumbicarbonate, and incidentally a number of impurities, as exemplied byTable II, initially has a measured alkalinity of 30 about 7.5 to 9.5.After it has been employed in Wood digestion and is recovered as blackliquor, its pH value is about 8.5 to 10.5. 4At no time during thecooking operation does the pH value fall below substantially 1 which isthe dividing line 35 between alkalinity and acidity. This is animportant feature of the invention since liquors which are below pH 7are considered as being acid and to have a corrosive action on steeldigesters.

I have found by experience that when the al- 40 kalinity is maintainedabove pH 7.5 there is a minimum of corrosion. It has been observed thatunder these conditions, a black adherent lm forms on the steel digester,and this film apparently aids in preventing the cooking liquor 45 fromattacking the underlying steel. It has further been observed that if theliquor has an inp. itial pH less than 7.5 the alkalinity drops to avalue during certain stages of the cooking, which permits corrosion inspite of the black nlm. Long 5o experience has shown that this loweringof the alkalinity during the first part of the cooking process is notpermanent, and that the alkalinity subsequently rises again during thecontinuance of the cooking operation. The rise is due in all 55probability to the escape of the acid constituent CO2, acetic acid andother gases, probably acidic in character. The decrease or drop duringthe initial stages may be explained by a variety of hypotheses, which,however, will be omitted. It is 60 suillcient to state that thealkalinity decreases and corrosion occurs if the initial alkalinity istoo low. Consequently, sufficient alkali carbonate is initially presentto give a pH of 7.5 or greater. A larger quantity may be used, butapparently no 65 additional protection is obtained.

The foregoing description of the cooking liquor has been directedgenerally tb that prepared from leach liquor. It is, of course, to beunderstood that it may be prepared by sulfiting a sodium car- 70 bonatesolution prepared from the commercial salt (NazCOa). For example, asolution containing eleven pounds of sodium carbonate (NazCOa) per cubicfoot is sulted by passing in sulfur dioxide (SOz) until there remainsonly 0.50 to 0.75 75 l :,oaas's pound of sodium bicarbonate (NaHCO:) perthebicarbonate is produced from the normal carbonate as an incident tothe sulting which cannot easily be avoided. However, when the salts aremixed without a sulting operation the cooking liquor may contain normalsodium carbonate Yand sodium sulte, the sodium bicarbonate not beingessential when the normal carbonate is' present.y

In the practical operation of the process it is most desirable toregenerate the liquor, but the process, chemically, is not dependentupon this type of liquor. A fresh liquor may be used when desired, andmay be made by any method which arrives at the desired proportions ofalkali carbonate and normal alkali sulflte. One such method is to sultea fresh sodium carbonate solution (NazCOs) until the requisite amount ofsoda remains unsulted. Another' method is to sulflte a pure, or acarbonate-containing, caustic soda. (NaOH), making up any deficiency incarbonate by an addition thereof.. Still another feature of the processis the flexibility of its recovery steps, not only to receive wastematerials from other processes,- but also to provide material for use inother processes. This permits conjoint operation of several distinctdigestion processes with a mingling of materials between them, andpermits, further the use of the same apparatus for the conjointlyoperated processes. Not only may the several processes be simultaneouslyoperated, but one may be readily followed by another because of thisadvantageous relation. Such a condition reduces the amount of equipmentrequired and-gives a Well equipped plant a wide flexibility in the useof apparatus and processes. For example,the soda process which employscaustic soda (NaOH) as the basis of the digestive liquor may utilize theleach liquor for causticizing with lime. The leach liquor, which is highin sodium sulde (NazS), may be causticized with advantage for thesulfate rocess. Thus, at any time when the leach liquor s deemed toohigh in sodium sulfide for the better requirements of .the presentinvention, it may be readily diverted into the sulfate process, withouteconomic loss, thus to elimivan acid sulflte process,having the samebase as used for the process ot this invention.

In the fore part of this description reference was made to certaindisadvantages of the soda or modied soda processes which are overcome bythe present invention, especially those using caustic soda and sodiumsulfitef One of these is the danger to humans of caustic solutions,which are very corrosive on the skin as compared to carbonates. Anotheris the diculty of preparing the sulflte-caustic soda solution.

On regeneration of cooking liquor for such caustic soda processes fromblack liquor the carbonate is formed. This carbonate requires inaddition to the sulting operation a causticizing operation. I eliminatethis causticizing step by retaining the carbonate unsuliited. Thecausticizing step is usually necessary even though in the preparation ofthe cooking liquor a caustic soda isv used initially instead ofcarbonate, be-

cause commercial caustic usually contains or readily forms a carbonateas impurity. It is customary to .causticize this impurity. Thesecausticizing operations -when conducted in the presence of alkalisulilte present technical difll- 5 culties, thedescription of which b eomitted as forming no part of this invention. When an attempt is madeotherwiseto 'eliminate the cans-'- ticizing operation, by over-suliltlnga soda solution and then adding caustic, anew condition is 1encountered. The sulting of the soda liberates carbonio acid gas (CO2)which remains dissolved in the solution. This gas can only be removedwith great operating dimculty and its removal is necessary for economyin the addition to the causl5 tic soda. Thus, the present invention,besides providing a new and advantageous pulping process, alsoeliminates causticizing operations and the C02 removal required by otherprocesses, and, furthermore, simplies the preparation of the 20 cookingliquor by the regeneration method.

The light color of the product is one of the prominent featuresrecommendingtlie use of this process. The black liquor contains 4acompound which 1s a chemical indicator, like litmus, cnang- 25y thelatter colors the pulp and the color cannot be readily washed out. Ifthe pulp is washed prior to a darkening'of the indicator, the latter isquite readily removed. Hence, to produce a light pulp it is important toprevent a high alkalinity 35 l in the presence of black liquor. Theelimination of strong caustic soda from the alkaline processes and itsreplacement by a milder carbonate contribute largely to the causes ofthe light color of the pulp. As a measure of the pulp color, the 40following examples are given in Table III, in which E is a sample ofunbleached tamarack,V and F is a sample of semi-bleached tamarack. Themethod used to determine the color consists in forming a sheet of thepulp sample and meas- 45 urlng the percent o f light reilected by thesheet at different colors, expressed as their wave lengths in ngstromunits.

Table II f Percent reiiection Wave length 55 460 (blue and indigo) 49 66iso 4s se 48 so 48 to 52 e1 57 65 60 5s o5 5s oo 61 6c sa 65 The aspenpulp is even lighter than the tama- 65 rack, above exempliiled.

It is to be understood that the process is not limited to use with onekind of wood, such as aspen, as herein described. ,Not only maydifferent woods of the same class, but woods of different classes may becooked as mixtures, such as mixed poplar and pine. -For example, I havesuccessfully cooked birch and tamarack. the former a short-ilberedhardwood, and the latter a longiibered conifer. The cooking operationisex- 75 tended tothe period requiredI for the slowest cooking woodwithout apparent damage to the pulp of the more rapidly cooking wood.'I'he pulp of such a mixture gives rise to certain operating advantages,especially when it is processed with a continuous filter discharging acake or heavy sheet of pulp froma suction roll. The long iibers aloneproduce a sheet which readily breaks, whereas the presence of shortfibers tends to strengthen the sheet by iilling in the smaller spaces.

Another object in mixing woods forcommon digestion is to improve. thecolor of the darker and stronger type of pulp by the presence of alighter and weaker type of pulp. For example, when ten to' twentypercent of birch is used with eighty to ninety percent of tamarack, theresultant pulp has been found to be as strong within commercial limits,as the tamarack pulp alone, and to have a color considerably lighterthan the tamarack pulp alone. It is not at all customary to mixshort-fibered hardwood pulp into longbered coniferous pulp for thispurpose. Not only does this hold true for birch and tamarack, but it hasbeen found to follow from mixed aspen and pine pulps.

It has also been found that mixed tamarack and pine, woods of the sameclass, have a strength disproportionately nearer that of the strongerpulp when both are made by my process, and that they have a strengthalmost proportionate when both are made by the sulfate or kraft process.

Other advantages of cooking mixed woods oi different classes will beobvious to those skilled in the art.

In the foregoing, I have described the process with reference to asodium base, but it is understood that the chemically equivalent basesor the mixed bases are contemplated, as will appear from the characterof the appended claims.

What I claim as new is:

1. The method of making substantially ligninfree cellulose pulp fromso-called hardwoods or short-bered wood which comprises digesting thewood until cellulose is released with an alkaline sodium sulfite liquor,containing suillcient excess of sodium sulte to prevent staining and asufficient amount of a sodium carbonate salt to give an alkalinity ofabout pH 7.5 to 9.5, at a temperature of from 175 to 188 C. and at thecorresponding saturated pressure for a period of about three to sevenhours, of which the flrst one to two hours is used in attaining saidtemperature and pressure.

2. The method of making substantially ligninfree cellulose pulp fromwood chips of the socalled hardwoods or short-bered woods whichcomprises digesting the wood until cellulose is released with analkaline sodium sulflte cooking liquor of an alkalinity not less than pH7.5, which solutionfcontains 30 to 40 pounds of sodium sulte and asuiicient quantity of a sodium salt of carbonic acid to produce therequired alkalinity 5 per pounds of oven dried chips, the concentrationof the liquor being such that 1/4 to cubic foot of liquor is used percubic foot of chips, at a temperature of from to 188 C., and at thecorresponding saturated steam pres- 10 sure for a period of from threeto seven hours, of which the first one to two hours is used in attainingsaid temperature and pressure.

3. The method of making substantially ligninfree cellulose pulp fromso-called hardwoods or 15 short-fibered wood which comprises digestingthe wood until cellulose is released with an alkaline sodium suliitecooking liquor of an alkalinity of a pH of about 7.5 to 9.5, whichliquor contains sufficient excess of sodium sulte to prevent 20staining, digesting it at a temperature of from 175 tol 188 C., and atthe corresponding saturated steam pressure for a period of about threeto seven hours, of which therst one to two hours is used in attainingtemperature and 25 pressure.

4. The method of making substantially ligninfree cellulose pulp fromwood chips of the socalled hardwoods or short-flbered woods whichcomprises digesting the wood until cellulose is 30 v released with analkaline sodium sulfite cooking liquor of an alkalinity not less than pH7.5, which contains 30 to 40 pounds of sodium sulte and `1.3 to 2.6pounds of sodium carbonate per 100 pounds of oven dried wood, theconcentration 35 of the liquor being such that 5/4 to 1%, cubic foot ofliquor is used per cubic foot of wood chips in the digester, at atemperature from 175 to 188 C., and at the corresponding saturated steampressure for a period of from three to seven hours, of which the irstone to two hours is used in attaining said temperature and pressure.

5. The process of making substantially ligninfree cellulose pulp fromso-called hardwood or short-bered wood which comprises digesting 45 thewood until cellulose is released, using a cooking liquor containing insolution from 30 to 40 parts of normal alkali metal sulflte per 100parts oi oven-dried wood so as to have suillcient excess ofvsaid sultepresent in solution to prevent staining wh'en the cellulose is released,maintaining the solution alkaline with a carbonate of an alkaliv metalthroughout the digestion, and continuing the digestion for from 3 to 7hours at an elevated temperature and pres- 55 sure of which the rst 1 to2 hours is used in attaining said temperature and pressure.

CLINTON K. TEXTOR.

